1. Field of the Invention
The present invention relates to data converters, and more particularly to delta sigma modulators.
2. Description of the Related Art
Analog to digital conversion is widely used in many circuit applications. Such application environments include hearing aids, cellular telephones, and other circuits which require conversion of audio information into a digital form. The single-loop higher order delta-sigma modulator based analog to digital converter (ADC) has recently found a significant application area in audio circuits such as these. Delta-sigma modulators combine the quantization noise reduction and eased analog circuitry benefits of oversampling ADC""s with the benefit of noise shaping to achieve a high resolution ADC.
Increased accuracy in ADC""s is continuously sought after so as to improve the performance of the circuit wherein the ADC is utilized. In addition to improved accuracy and resolution, power consumption in battery powered devices implementing a delta-sigma converter can be of significant concern.
A delta-sigma modulator employing a single loop, fourth order architecture comprises an input signal section, an output signal section, four integration stages connected in a cascaded configuration and having a signal path, at least one feedback path from the output signal section, and a feed-forward path, connected to the input signal section and to the signal path prior to a fourth integration stage. The feed-forward path includes a differentiator, which is connected to the signal path prior to the fourth integration stage. The delta-sigma modulator can be implemented in, for example, an analog to digital converter.
In one embodiment, at least one of the integration stages has a ratio of gain bandwidth product to sampling frequency of less than about 0.2 times a resolution of the delta-sigma modulator.
A method of improving error processing in a fourth order delta-sigma modulator having four integration stages comprises connecting an input signal feed-forward path to the signal path prior to a fourth integration stage. The method further comprises differentiating an input signal along the feed-forward path.
A fourth order delta-sigma modulator, for use in an analog to digital converter, comprises an input signal section, an output signal section, a plurality of integration stages connected in a cascaded configuration, a feed-forward path coupled to the input signal section and connected to the signal path prior to a last of the plurality of integration stages. The feed-forward path includes a differentiator.
According to another aspect of the invention, an audio circuit comprises a fourth order delta-sigma modulator having a feed-forward path connected from an input signal section to the signal path prior to a final integration stage. In one embodiment of the audio circuit, the feed-forward path includes a differentiator. In another embodiment, wherein the modulator comprises a plurality of integrators, at least one of the integrators has a ratio of gain bandwidth product to sampling frequency of less than about 0.2 times a resolution of the modulator.